<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .d1 {
            border: 1px solid #000;
            width: 100px;
            height: 100px;
        }

        .d1::after {
            content: '';
            width: 0;
            height: 100%;
            display: inline-block;
            vertical-align: middle;
            border: 1px solid red;
        }

        .d2 {
            display: inline-block;
            vertical-align: middle;
            border: 1px solid blue;

        }
    </style>

</head>

<body>
    <div class="d1">
        <div class="d2"></div>
    </div>

</body>

</html>